Current and voltage transducers are known to be useful by utilities for monitoring and recording currents and voltages at various locations within their supervisory control systems, as well as being useful in other apparatus, including, for example, industrial control applications.
Typical current and voltage transducer devices now known have nominal inputs of 5 amperes and/or 120 volts and nominal outputs of 1 milliampere DC (current transducer) or 1 milliampere at 150 volts (voltage transducer), with most (over 90%) of such devices being of the average responding RMS indicating type. The average responding RMS indicating type of transducer responds to the average value of the AC signal rather than to the root means square value, but is calibrated such that the output signal corresponds to the root means square value of the input signal. In addition, such a transducer is more economical than true RMS devices and has proved to be quite satisfactory in fulfilling the majority of needs.
With respect to prior art patents, U.S. Pat. No. 3,971,979 is directed to a current/voltage transducer that includes a current mirror circuit having reference and output legs each of which includes at least one transistor and associated emitter and/or collector resistances.
In this type of device, the base emitter voltage of the reference transistor and output transistor directly enter into the transfer function and their drop affect the gain of the circuit, as does the emitter connected resistor. As a result, the circuit must be adjusted (as, for example, by a potentiometer included in the reference leg of the transducer shown in U.S. Pat. No. 3,971,979), and requires periodic recalibration due to changes in the values of components utilized.
Since the transistors and resistors in the circuit are connected to the secondary winding of the transformer, the values of these components influence the effective secondary resistance which is in series with the resistance in the primary winding circuit of the transformer for voltage transducers, which likewise affects calibration and calibration stability. In addition, the primary winding of a voltage transducer is normally made of copper the resistance of which changes due to temperature changes and this creates a temperature dependant factor in calibration of the device. The secondary winding is also temperature dependant to some degree and the available winding area on the core must be divided to optimize temperature drifts due to the copper windings.